Since Meteor
Crater in Arizona was first discovered and recognized as an impact crater in
the early 20th century, scientists have puzzled over why they could not find
much melted rock at the site. At most impact locales, such as the Chicxulub
crater in Mexico, a distinct layer of melted rocks indicates heat generated
from a high-velocity impact. New findings, however, suggest that rather than
one large meteorite striking the ground at a high velocity, a lower velocity,
pancake-shaped swarm of meteorite pieces  formed from the explosion a
larger meteorite  likely carved out Meteor Crater.
Scientists now say that Arizonas Meteor Crater may have been formed by
a swarm of meteorite pieces rather than one large meteorite, which may help
to explain why the craters geology differs from that of other impact sites.
Image Peter L. Kresan, copyright 2005.

The idea of a swarm of meteorites forming the crater had been floated by other
researchers in the past, says Bevan French of the Smithsonian Institution in
Washington, D.C. But their ideas never went far enough to answer
the questions, he says. In the new study, published in the March 10 Nature,
Jay Melosh and Gareth Collins bridge the modeling and geologic work to
find the answer, French says.

Last summer, Melosh, of the Lunar and Planetary Laboratory at the University
of Arizona in Tucson, and Collins, of Imperial College in London, were playing
with their public-friendly Web site model of what happens when extraterrestrial
objects hit Earth, when they decided to plug in the information they thought
they knew about Meteor Crater. Our results showed that Earths atmosphere
drastically affected the meteorite. At first we thought it had to be a mistake,
but after several recalibrations of the model, we found it was right.

About 55,000 years ago, a 40-meter diameter iron meteorite streaked toward present-day
Arizona on a collision course. According to Melosh and Collins model,
at about 14 kilometers above Earth, the atmospheric pressure caused the meteor
to shatter into pieces, which spread out to about 100 meters in diameter. The
cluster of meteorite pieces spread slowly, limited by the low lateral
acceleration of the massive iron fragments, Melosh says. Aerodynamic
drag forces continued to urge the pieces apart while slowing the clusters
overall velocity. By the time it reached 5 kilometers above land, the
swarm was at least 200 meters across. When the swarm struck land, it had lost
80 percent of its energy and was probably about 300 meters wide, Melosh says.

According to the models, the surface-impact velocity was probably about 12 kilometers
per second  far less than the 15 to 20 kilometers per second widely assumed,
he says. Nonetheless, the impact set off the equivalent energy of a 2.5-megaton
blast of TNT, and left behind a crater that is 1.2 kilometers wide and 174 meters
deep.

This explanation makes sense, French says, and is consistent with an absence
of large amounts of melt in the crater. An important thought that comes out
of this research though, he says, is that an object this small could cause so
much damage. Thus, it is important to model other potential impacts with small
projectiles to see what would happen in populated areas.